Abstract: CH₄ is a typical and special detonation mixture. Based on the experimental method, digital image processing was performed to study the detonation stability theory of methane. A premixed CH₄ + 2O₂ mixture was ignited in a tube with an inner diameter of 50.8 mm under different initial pressures. Smoked foils were used to record the cellular structure of spinning detonation. The average detonation velocity measured is similar with the CJ detonation velocity and it demonstrates that steady detonation happens when the initial pressure is higher than 5 kPa. The triple point trajectory leaves a very irregular pattern in smoked foils. In order to decrease human error, the digital processing technology was utilized and improved. The CH₄ + 2O₂ mixture shows high degree of irregularity in the computation of histograms and the autocorrelation function. The cellular size and gap of the unstable premixed CH₄ + 2O₂ were given. It is found that the results measured by digital image processing are accurate and by eyes are too big. The research can calculate the cellular size and the degree of instability of the unstable premixed CH₄ + 2O₂ and improves the quantitative irregular calculation method.